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Electric Potential Driven Pressure Sensing Observation in new Hollow Radial ZnO and their Heterostructure with Carbon Radhamanohar Aepuru, and Himanshu Sekhar Panda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00123 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 19, 2016

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The Journal of Physical Chemistry

Electric Potential Driven Pressure Sensing Observation in new Hollow Radial ZnO and their Heterostructure with Carbon Radhamanohar Aepuru and H S Panda* Department of Materials Engineering, Defence Institute of Advanced Technology, Pune 411025, India

*Corresponding Author’s Address: Tel. No.: +91-20-24304205 Email: [email protected], [email protected] Department of Materials Engineering, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India

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ABSTRACT Many applications demonstrated for ZnO desire high dielectric constant and fast induced polarization properties with frequency. We present, new hollow radial zinc oxide nanostructure and carbon decorated hollow ZnO heterostructures, which greatly influence the dielectric and capacitive pressure sensing properties after integrating in polyvinyldine fluoride matrix. The hollow zinc oxide nanostructure was realized by using a simple and cost-effective ambient temperature template-free synthesis process. Crystal structure was investigated by using X-ray diffraction and Raman spectroscopy, and then correlated with high resolution transmission electron microscopy, which reveals buckling of lattice fringes. Mimicking the wurzite structure of ZnO, the developed carbon decorated hollow radial ZnO improved the dielectric constant ~140 at 100 Hz and reinforcing efficiency >4 times than hollow radial ZnO due to strong interfacial polarization. We demonstrated a simple design, fabrication and testing of flexible polymer composite pressure sensing device by using such fascinating nanostructures. The capacitive pressure sensing response is calculated to be 35 times (≤ 0.002 MPa) and 24 times (≤ 0.006 MPa) higher than the pristine PVDF based device after poling under corona. Further, the developed devices showed significant capacitive change upon bending due to the displacement of electrodes and change in spacing between the fillers in the polymer matrix.


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1. INTRODUCTION The demand of flexible electronic devices owes the scientific community to develop novel electronic materials for various energy storage/conversion applications, including sensors/ actuators, super-capacitors, energy harvesters and nano-generators.1-7 Flexible polymeric devices are demonstrated earlier to a certain extent by using poly(vinylidene fluoride) (PVDF), poly(dimethylsiloxane) (PDMS) and poly(methyl methacrylate) (PMMA).8-11 However, these polymeric materials posses low electrical properties than metal oxides. Consequently, polymer and inorganic metal oxide filler composites have drawn a lot of attention and showed synergetic effect on electrical and mechanical properties.12 Among the class of inorganic fillers, zinc oxide (ZnO) has captured huge attention because of broad applications ranging from sensors to nano-generators. The wide band gap (3.3 eV) and large exciton binding energy (60 meV)13 of ZnO create an unique semiconducting and piezoelectric material for versatile electronic and optical devices. In recent years, preparation of various shapes ZnO such as spherical, nano-rods, radial, and flower-like were reported and incorporated in the polymer matrix for improving the electrical properties.14-16 However, there is no report on the preparation of hollow radial ZnO and their effect in polymer matrix. The hollow nanostructures are beneficial as it governs the desired properties due to high active surface area. In general, hollow nanostructures are realized by template-free and template-assisted methods.17 Most of the reported hollow nanostructures are spherical, however due to the processing difficulties, limited literature available on the synthesis of non-spherical hollow structures.18 The hollow formation in nanostructures was accounted by well-known Ostwald ripening mechanism17,18 (especially for template-free methods) that greatly elevated the ability to control the properties like mechanical, electrical, optical and chemical, which in turn explored to diverse 3 ACS Paragon Plus Environment


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fields viz. energy storage, sensing and biomedical applications.18 In another aspect, progress is ongoing to control the electrical properties of ZnO by incorporating defects/impurities in the lattice sites, thereby improves the desired properties. Although, a lot of work is done on ZnO and doped ZnO19-21, to the best of our knowledge very little work has been focused on the development of hybrid ZnO heterostructures by using inorganic/organic materials. Carbon is one of the most important and widely used organic materials found in various applications because of outstanding physical and chemical properties.22,23 Recently, carbon materials such as carbon nanotubes (CNTs), activated carbon, reduced graphene oxide (RGO) and graphene are used as active materials to deposit on the surface of metals, polymers and metal oxides.24,25 Also, these active organic materials are used as fillers for developing composite materials.26,27 The electron transport mechanism of carbon makes it suitable for improving the stability, thermal and electrical properties, and especially for developing hybrid materials by depositing carbon on the metal oxides. For instance, Hossian et.al fabricated the cylindrical microtowers of ZnO/C core shell hexagonal nanorods by using thermolysis route and the electrical conductivity of microtowers was measured to be 1.6 - 63 S/m.28 Dong et al. demonstrated the synthesis of ZnO-graphene quantum dots for optoelectronic devices and fabricated a multilayer polymer-quantum dot LED device by using spin coating method.29 Shuaishuai et al. reported the fabrication of ZnO/C composites by depositing carbon on the surface of ZnO by adsorption and calcination process and studied the photocatalytic properties.24 Recently, Chen et al. investigated the capacitive pressure response of a PMMA-ZnO nanowire dielectric composite based device. The composite exhibits effective pressure response than pristine polymer due to geometry change in the capacitor and/or induced polarization of piezoelectric nanowires.11 However, the effect of stress induced polarization contribution of ZnO 4 ACS Paragon Plus Environment

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has not testified and correlated with the capacitive sensitivity. Therefore, we hypothesize to develop hollow radial ZnO and carbon-ZnO hybrid heterostructure and their composites with PVDF for elevating the electrical properties and the capacitive pressure sensing performance of the device by investigating the stress induced polarization contribution of zinc oxide. In this article, hollow radial ZnO (HZnO) was synthesized by using a new user friendly template-free chemical process. HZnO is functionalized with carbon (CZnO) and both used as filler in PVDF matrix separately for making superior dielectric and flexible pressure sensing device. HZnO and CZnO heterostructures were investigated by using various sophisticated analytical techniques to confirm the crystal structure, hollow morphology, and deposition of carbon on ZnO. The dielectric properties of the PVDF-HZnO and PVDF-CZnO nanocomposites were measured at different frequencies and temperatures and correlated with a proposed model. The results revealed that the PVDF-CZnO nanocomposites leads strong interfacial polarization, and showed enhance dielectric properties under applied electric field. Further, developed nanocomposites were used for making flexible capacitive pressure sensing device in poled, unpoled and bending condition to meet the practical application. 2. EXPERIMENTAL SECTION 2.1 Materials and Methods: Template-free synthesis of hollow radial ZnO (HZnO) nanoparticles: Hollow radial ZnO nanoparticles were synthesized by using a new, facile and cost effective template-free chemical process. Initially, 3.285 g of Zinc acetate dihydrate (Merck, ≥ 98%) was added in 15 ml Hydrazine hydrate (Merck, ≥ 99% purity) and sonicated in a bath sonicator for 15 mins. Thereafter, the white precipitate obtained was quenched by adding de-ionized water and 5 ACS Paragon Plus Environment


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sonicated for another 5-10 mins. After that, the white precipitate was disappeared and reappeared when the sonication process was extended further for 20-25 mins. The precipitate was harvested and washed with de-ionized water and ethanol several times, and dried at 50 °C for 12 h. Synthesis of carbon-ZnO hybrid heterostructures (CZnO): Various concentrations (0.3 mg – 2 mg) of carbon nanoparticles (superconducting, 99+% ALFA ASSER) was added in hydrazine hydrate (15 ml) separately and sonicated in a water bath sonicator for 15 mins. Then, zinc acetate (3.285 g) was added to the carbon nanoparticles solution and sonicated for another 15 mins. After that, deionized water was added to the above solution and continued sonication for another 30 mins. The precipitate was separated, cleaned and dried by following the hollow radial ZnO process. Preparation of PVDF, PVDF-HZnO and PVDF-CZnO nanocomposite films: The composite films were fabricated by using a solution casting technique as reported earlier by us.14,30 In short, PVDF (sigma Aldrich) was dissolved in Dimethylacetamide (DMA) under continuous stirring. Different concentration of CZnO (0 to 40 wt %) were dispersed in DMA and transferred to the above PVDF solution separately. The composite solution was stirred and sonicated to obtain homogeneous dispersion and thereafter poured into a glass dish (dia. 96 mm) and dried in the oven for overnight at 65 oC. Similar procedure was followed to prepare PVDF-HZnO and pure PVDF films. Fabrication of capacitive pressure sensing device: The developed composite films were cut in to 20 mm × 20 mm in size. The surface of the composite films was sputter-coated with Au electrode on both sides. Thereafter, the films were sandwiched with conducting Cu tape to give electrical contacts. The device was covered with kapton (polyamide) tape both sides for making


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it robust. The device was poled under corona with applied voltage of 15 kV and temperature 60 o

C for 1 hour.

2.2 Characterization: Relaxation studies (T2) of HZnO and different CZnO hybrid heterostructures were carried out separately to obtain the optimum concentration of carbon nanoparticles adsorption on HZnO surface by using a XIGO Arcon surface area analyzer. The crystal structure of HZnO and CZnO nanoparticles was determined through powder X-ray diffractometer (BRUKER D8 ADVANCE) with Cu-Kα radiation (λ = 1.542 Å) with scanning angle (2) of 10-80o. Raman study was carried out for the developed materials by using Raman spectroscopy (Lab RAM HR (800), HORIBA Scientific with Olympus BX41) having solid state laser excitation wavelength 632 nm. The morphology of the HZnO and CZnO was obtained by using High resolution transmission electron microscope (FEI Tecnai G230, Hillsboro, USA) and Field emission scanning electron microscope (SIGMA, Carl Zesis). Electron spin resonance (ESR) spectra was recorded at room temperature by using ESR X-band spectrometer (JEOL, JES-FA200), operating at frequency 9.45 GHz, power 2.99 mW, sweep time 2 mins, field center 336 mT, width +/- 100 mT and modulation frequency 100 kHz, to know the carrier mobility and adsorption of carbon on ZnO, by detecting the available unpaired electrons in the HZnO and CZnO hybrid heterostructures. Broadband Dielectric Spectroscopy (BDS) studies were performed to measure the complex dielectric permittivity and electric modulus of composite samples using a Novocontrol broadband dielectric spectrometer with Alpha-A analyzer interfaced to the sample cell equipped with temperature controller and WinFit data analysis software. The measurements were carried out in the wide range of frequency from 10 MHz to 1 Hz over the temperature (25– 90 oC). The samples were ensured by proper electric contact by quick drying silver paste and placed between 7 ACS Paragon Plus Environment


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gold plated electrodes (dia. 20 mm). The capacitive pressure sensing response was investigated by measuring the capacitance of the device with respect to the applied load at frequency 1 KHz. 3. RESULTS AND DISCUSSION Formation and growth mechanism of hollow radial ZnO nanostructures: The synthesis route for the formation of hollow ZnO nanostructures follows hydrolysis of constituent precursors and poly-condensation. In this process, zinc acetate is used as Zn2+ cation and Hydrazine hydrate as OH- anion precursor source for the formation of ZnO nanostructures. For the growth mechanism, Hydrazine hydrate plays a vital role by providing anion source and builds chemical equilibrium between hydroxyl (OH)- and (N2H5)+ ions in the solution as represented in equation (1). In the aqueous media (Path-I), Zinc acetate dihydrates undergo hydrolysis as in equation (2a) and formed Zn(OH)2 and acetic acid. Further quenching with excess H2O, Zn(OH)2 reacted with hydroxide ions and formed [Zn(OH)4]2- ions as in equation (3), which interchanged to ZnO under the localise heat generated by sonication. In another pathway (Path-II), Zinc acetate reacted with hydrazine hydrate through exothermic reaction and formed highly unstable zinc hydrazinium complex [Zn(N2H4)2]2+ as shown in equation (2b). The complex ions due to unstable nature interchanged with hydroxyl ions and formed Zn(OH)2 and hydrazine (N2H4) as shown in equation (2c). Further quenching with H2O, Zn(OH)2 was dissolved completely and formed colourless transparent solution of [Zn(OH)4]2- ions (equation 3). The free hydrazine which is present in the solution, acted as a capping agent to zinc hydronium ions for controlling the nucleation and growth process of ZnO nanostructure. The reaction mechanism of the synthesis process is given below:


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The radial hollow ZnO formation mechanism involves sequential steps: generation of nanocrystallites; aggregation and self assembly; and hollowing process due to inside-out Ostwald ripening caused by surface energetic factors.18,31,32 The schematic diagram of the growth process is shown in the Figure 1. In the sonication process, ZnO nanocrystallites are nucleated and grown due to the localized heat generation. These nanocrystallites having high energy aggregated and self assembled to minimize the surface energy. Prolonging the sonication time, spontaneous inside-out Ostwald ripening process occurs. As a result, loosely packed nanocrystallites (in the inner region) possessing higher surface energy have strong tendency to dissolve and might redeposit or form new crystallites. During the reaction, ZnO nuclei formation was facilitated in the initial growth stage preferably along the [001] direction due to the localized heat generation by sonication. The higher growth rate along [001] direction and the effect of capping agent plays a key role for developing radial rod like ZnO nanostructures.18,31 The proposed growth process was confirmed also by microscopic observations. In a similar manner, ZnO hollow structures (nut-, candle- and sphere-like) were reported recently by aging the nanostructures at room temperature and also by sonochemical process.31,32 9 ACS Paragon Plus Environment


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Figure 1. Schematic representation for the formation of hollow radial ZnO

Figure 2. (a) Relaxation studies of CZnO heterostructures with various concentration of carbon, (b) X-ray diffraction patterns of HZnO nanostructures and CZnO heterostructures, (c) Raman spectrum and (d) ESR spectrum of CZnO heterostructures in comparison with HZnO nanostructure and carbon nanoparticles.


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Carbon nanoparticles adsorption study on hollow radial ZnO nanostructures: Organic ligands are often used in conjunction with metal oxides for giving stable homogeneous dispersion in the solvent. However, we are demonstrated the carbon adsorption on the surface of hollow radial ZnO by conducting a solvent relaxation experiments, which is based on the time domain nuclear magnetic resonance, which measured the relaxation time (T1 & T2). The relaxation time of the bulk liquid is different than that of the liquid on the particles surface and the change in the relaxation time determined the competitive adsorption of materials onto the surfaces. Spin-Spin relaxation rate (R2) data of bound liquid was recorded for various concentrations of carbon (0-2 mg) and the specific relaxation rate enhancement (R2sp) was estimated by using the following formula:33,34 = −1

(4)

R2 and R2bulk are the relaxation rates of bound liquid in CZnO (Carbon concentration ranges from 0-2 mg) and bulk liquid (ethanol) respectively. The relaxation time (T2) was measured in CPMG (Carr−Purcell−Meiboom−Gill) mode with 180o pulse with τ = 0.5 ms. The increase of carbon concentration in CZnO induced a decrease in specific relaxation rate as shown in Figure 2(a), because the solvent displaced away from the ZnO surface and resulted a new interaction of carbon with the free solvent having lower surface relaxivity. The relaxation rate decreased in CZnO up to a critical adsorption limit of carbon and thereafter reaches saturation. The relaxation study is practically assistive to get the optimum loading and found to be ~1.5 mg. Structural investigations: X-ray diffraction patterns of HZnO and the CZnO nanoparticles are shown in Figure 2(b). All the diffraction patterns were indexed as wurzite structure of ZnO, and



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are good agreement with standard JCPDS data (JCPDS No. 89-1397) and the lattice parameter values were calculated to be ‘a’~ 3.25 Å and ‘c’ ~ 5.21 Å with c/a ratio of ~1.603, and confirmed the ideal axial ratio of hexagonal close packed structure. However, after adsorption of carbon on HZnO the diffraction peaks (002) and (101) of the wurzite structure are shifted towards higher two theta (inset of Figure 2(b)). The shifting of the diffraction peaks towards higher two theta suggested the generation of compressive stress in HZnO with the addition of carbon due to carbon-HZnO interaction. Again, the lattice parameters of CZnO were calculated to be ‘a’~ 3.235 Å and ‘c’ ~ 5.182 Å with slight decrease in axial ratio (c/a ratio ~1.601). The Raman spectra of the carbon nanoparticles, HZnO and CZnO are presented in the Figure 2(c), to know the chemical interaction of the HZnO and carbon in the heterostructures. The Raman spectra of the carbon showed the two peaks around 1336 cm-1 (D band) and 1595 cm-1 (G band). The raman active E2g phonon mode at 1595 cm-1 suggests the presence of sp2 hybridized carbon, whereas the peak at 1336 cm-1 corresponds to the existence of defects in the sp2 hybridized disordered carbon layers.28,35 The spectrum also suggests the existence of E2 (high) phonon mode at 436 cm-1, which is attributed for Zn-O stretching of wurtzite ZnO.29 The raman spectrum of ZnO evidence weak peaks at ~ 338 cm-1 (E2H-E2L multiphonon mode), a shoulder peak ~ 410 cm-1 (E1TO mode) and 583.3 cm-1 (E1L phonon mode engendered by defects and impurities).36 The broad peak at 1091 and 1155 cm-1 are assigned to be 2LO mode; 2A1L, 2E1L phonon modes. Also, ZnO heterostructure (CZnO) evidences all the three major characteristic peaks. However, the weak Raman peaks (338 cm-1 , 583.3 cm-1,1091 cm-1 and 1155 cm-1) are absent in CZnO due to increase in disorderness caused by carbon. Also, the E2 phonon mode peaks of CZnO shifted ~ 8 cm-1 than compared to HZnO. The peak is blue shifted to 428 cm-1 in CZnO compared to HZnO (436 cm-1) nanostructures. The G band peak of carbon at 1595 12 ACS Paragon Plus Environment

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cm-1 is blue shifted to 1589 cm-1 in the CZnO. The shift in the peaks is considered to be due to relatively change in the size, aroused by the local stress with the incorporation of carbon leading to distortions, thus suggesting the interaction with ZnO in the heterostructures. The interaction of ZnO and carbon is further confirmed by ESR and morphology studies. ESR study: To know the interaction of carbon on ZnO in CZnO heterostructures, electron spin resonance (ESR) technique was applied for studying the absorbed species (free radicals, unpaired electrons) and defects and shown in Figure 2(d). ESR technique is based upon the spin interactions of free electrons having the magnetic components ms= ±1/2 under the influence of external magnetic field with field strength Bo and specific energy. Thus, the free electrons have two energy levels that move between them by either absorbing or emitting electromagnetic radiation. The change in the energy levels of the electron is expressed by ∆E = gBoμB, where μB is the Bohr magnetron (9.274×10-24 JT-1) and g is the g-factor of the electron, which is calculated by g = hν/μBBo.30,37 The ESR study revealed strong ESR resonance between hollow CZnO heterostructures and HZnO nanostructures. Single derivative peak was observed in carbon and HZnO each, where as splitting of the derivative peak was evidenced in the CZnO heterostructures, suggesting the hyperfine interactions. The doublet in the CZnO heterostructures is caused by the interactions of the unpaired electrons of HZnO with the neighbouring nuclei. The intense ESR signal at 337 mT suggested the high availability of unpaired electrons in heterostructures. Microscopic studies: With relevant to solvent relaxation and structural investigations, microscopic studies were carried out for developed HZnO and CZnO, and shown in Figure 3. TEM and SEM images (Figure 3(a) and 3(b)) of HZnO suggested the formation of hollow three dimensional radial zinc oxide nanostructures. The average diameter of the hollow part of the 13 ACS Paragon Plus Environment


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HZnO nanostructures was estimated to be ~ 80 nm. Figure 3(c) showed the TEM image of CZnO heterostructures and confirmed the adsorption of carbon on HZnO surface. Further, high resolution TEM image revealed the incoherent lattice distortion of HZnO during formation due to the carbon adsorption.

Figure 3. TEM images of (a) HZnO nanostructure (c) CZnO heterostructures; (b) FESEM image of HZnO nanostructures with inset showing at high magnification; (d) High resolution TEM images of CZnO heterostructures with inset showing the surface profile plot (left) and magnified image of marked area (right). The hollow diameter of the CZnO heterostructures were decreased and calculated to be ~ 40 nm and the size of carbon nanoparticles are ~ 40 nm (Figure 3c and Figure S1, Supporting 14 ACS Paragon Plus Environment

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information). High resolution TEM image (Figure 3(d)) showed the inter-planar spacing of 0.52 nm (inset shows the surface profile plot) along [001] direction, which corresponds to (0001) plane of ZnO. The distortion in the lattice planes of ZnO might be due to the interaction of carbon on ZnO, which buckles the lattice fringes and resulted strain along growth direction as shown in the inset of Figure 3(d). The strain in the lattice was supported also by X-ray diffraction. SEM images and energy dispersive X-ray analysis data of the CZnO heterostructures are shown in the supporting information (Figure S1 and S2). The elemental composition of ZnO heterostructures suggests the existence of constituent elements viz. major concentrations of Zn, O and minor concentration of carbon. Dielectric studies: The frequency dependent real part of dielectric permittivity of the PVDF and PVDF based composites (PVDF-HZnO, PVDF-CZnO) was measured at room temperature and shown in the Figure 4(a). As envisioned, all the composites (PVDF-HZnO & PVDF-CZnO) showed enhance dielectric constant than pure PVDF (10 at 100 Hz). Further, the dielectric constant achieved in PVDF-CZnO nanocomposite significantly higher than PVDF-HZnO nanocomposite and earlier reports.16,30 The dielectric permittivity of PVDF-HZnO composite was 30 at 100 Hz, and increased to 142 in PVDF-CZnO composite at the percolation threshold. The dielectric permittivity of CZnO with various concentrations of carbon (0-2 mg) was shown in the supporting information (Figure S3). Results revealed the increase of dielectric constant with increase in carbon content up to 1.5 mg and thereafter decreases, which is in accord with the relaxation studies discussion (Optimum concentration).



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Figure 4. Frequency dependent (a) dielectric permittivity of developed films, (b) Reinforcing efficiency plots showing contribution of fillers in polymer matrix, (c) Dielectric loss tangent and (d) Imaginary part of electrical modulus of developed composite films. Also, dielectric permittivity of PVDF-CZnO (0 to 40 wt %) composite films was shown in the supporting information (Figure S4). The significant increase in the dielectric permittivity might be the strong polarization induced in the CZnO heterostructures due to the high availability of charge carriers. The dielectric permittivity of the PVDF based composite increases, because of the formation of radial ZnO microcapacitors in the composite structure as reported earlier.16 However, the percolation threshold limit of our developed composites was observed at higher filler concentration (≥ 35 wt %). Therefore, a model is proposed to explain the 16 ACS Paragon Plus Environment

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enhancement in the dielectric properties and threshold limit of PVDF-CZnO composite film and shown in a schematic model (Figure 5).

Figure 5. Pseudo-microcapacitor model of hollow radial CZnO heterostructures As proposed in the model, the carbon nanoparticles are decorated both side of the hollow radial ZnO, and acting as surface electrodes, where ZnO is a dielectric medium. As a result, several individual hollow radial ZnO pseudo-microcapacitor structures were formed, hence influenced the overall performance of the microcapacitor structure in the PVDF-CZnO composite film, and enhanced the dielectric and threshold limit values. The dielectric permittivity was decreased steeply with increasing the frequency, showed strong frequency dispersion at lower frequencies. This phenomenon is best explained by using the induced polarization mechanism in the PVDF composites. At higher frequencies, the dipoles do not have adequate time to respond and showed dominant orientational polarization, which leads to decrease in the permittivity. Whereas, at lower frequencies space charge/interfacial polarization occurred between the polymers and fillers.30,38-41 As predicted in the pseudo-microcapacitor model,

the interaction as well as

contribution of fillers for polarization in the polymer matrix was analyzed by calculating the dielectric reinforcing function (DRF) and is defined in equation (5). =

ԑ )

(5)

ԑ !" ) ‫׳‬



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DRF is a measured of the normalize polarization by the matrix, thus the reinforcing efficiency of the heterostructures alone and their level of energy storage capacity can be determined. Figure 4b confirmed the polarization contribution of CZnO heterostructures is much higher than HZnO nanostructures, as the dielectric permittivity is proportional to polarization. At lower frequency (100 Hz), the polarization contribution (reinforcing efficiency (G)) caused by the CZnO and HZnO fillers is 20 and 5 respectively. Thus, the induced polarization in CZnO heterostructures is 4 times higher than HZnO nanostructures. Similar observations were accounted for epoxycomposites earlier.42 The dielectric permittivity of our developed composites is significantly higher than earlier reported PVDF-ZnO composites.15,16,30,39 In accordance with the ԑ′ , the dielectric loss tangent (tan δ) of the composites were recorded as a function of frequency. The dielectric loss is related to the complex permittivity and is expressed as: ԑ∗ = ԑ′ − $ ԑ′′

(6)

Where ԑ′ and ԑ′′ are the real and imaginary parts of complex permittivity, and the dielectric loss tangent is%&' ( =

ԑ ԑ

(7)

Figure 4(c) suggested the increase of dielectric loss with the addition of fillers in the PVDF matrix. The loss tends to decrease with increasing frequency and considerably higher at lower frequencies, which is due to increase in conductivity and the induced relaxation caused by the polarization effects or Debye losses.43 To study the relaxation behavior, imaginary part of the electric modulus was recorded and shown in Figure 4(d). The relaxation peak was shifted 18 ACS Paragon Plus Environment

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towards higher frequencies in the composite films when compared to pure PVDF film, which is due to the interfacial polarization also known as Maxwell−Wagner−Sillars (MWS) polarization. The origin of MWS polarization in the composite film is due to the accumulation of charge carriers at the interface regions of fillers and matrix. However, the MWS polarization in pure PVDF film at the lower frequencies is due to the charge carrier accumulation between the amorphous and crystal regions. Again, the relaxation mechanism in PVDF composite films is promoted for temperature dependent dielectric studies.

Figure 6. Frequency dependent dielectric permittivity of (a) PVDF-HZnO (b) PVDF-CZnO and imaginary part of electrical modulus of (c) PVDF-HZnO (d) PVDF-CZnO composite films measured at various temperatures.



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Temperature dependent dielectric studies of PVDF-CZnO (35 wt %) composite films were performed and compared with PVDF-HZnO (35 wt %) composite films (Figure 6). The dielectric permittivity of PVDF-CZnO film (figure 6(b)) at 0 oC was ~ 110 and tended to increase with temperature, attaining 800 at 100 oC. The remarkable effect of temperature on the dielectric permittivity is due to increasing the conductivity of the hollow CZnO heterostructures and the thermal expansion of PVDF matrix. On the other side, the permittivity of PVDF-HZnO (35 wt %) composite film was achieved around 90 at 100 oC as evidenced in Figure 6(a). Further, the temperature dependent a.c conductivity was measured and the conductivity value is in good agreement with the dielectric studies and shown in Figure S5 (Supporting information). The a.c. conductivity was increased from 1.1 × 10-9 S/cm to 7.2 × 10-7 S/cm, when the temperature changes from 0 to 100 oC. The proportionate increase is due to the temperature dependence interfacial polarization, which is inferred by the addition of the semiconducting fillers in the polymer matrix. The induced interfacial polarization in the composite films was further investigated by temperature dependent electric modulus studies, and is shown in Figure 6(c) & 6(d). As noticed in the frequency dependent electric modulus (Figure 4(d)), the PVDF-CZnO composite film showed similar relaxation behavior towards higher frequency with increase in the temperature from 0 to 100 oC. The simultaneous shift of the relaxation peaks is the typical characteristics of MWS polarization and is good agreement with the earlier literature.30,43 Thereafter, the activation energies of the developed composite films was calculated and shown in Figure 7. The activation energy describes the minimum energy required for the charge carriers to accumulate at the interfacial regions. The activation energies of developed composites were determined by using Arrhenius equation: /

) = )* +,- .− 01 20 ACS Paragon Plus Environment

(8)

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Where f and T are the frequency maximum and temperature in electric modulus, Ea is the activation energy determined from the slope and k is the Boltzmann’s constant. The developed composite films exhibited Arrhenius type relaxation behavior. Thus, the activation energies (Ea) for PVDF-HZnO and PVDF-CZnO composite films are calculated to be 0.26 eV and 0.45 eV respectively. The increase in activation energy is due to impeding the orientation of electric dipoles by decreasing the mobility of the polymer with the addition of ZnO hetrostructures suggesting strong interfacial effect than compared to HZnO nanostructures. Similar reports of activation energy showing relaxation behavior was observed earlier for PVDF based composites.40

12

PVDF-HZnO nanostructures PVDF-CZnO heterostructures

11 10 ln (Fmax)

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Ea=0.45 eV

9 8 Ea=0.26 eV

7 6 0.0028

0.0030

0.0032 1/K

0.0034

0.0036

Figure 7. Activation energies plot of PVDF based composite films. After successful investigation of physical and electrical properties, the developed composite films were used to fabricate a capacitive pressure sensing device. The films were sandwiched between the metal electrodes and acted like a parallel plate capacitor. The schematic representation of a developed device is shown in the Figure 8. 21 ACS Paragon Plus Environment


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Figure 8. Schematic representation of a capacitive pressure sensing device having flexible nature.

The capacitance was measured at various static load conditions by using dielectric analyzer. In percepts, the capacitance is comparative to the spacing between the parallel metal electrodes. Thus, the change in capacitance with respect to load caused the reduction of the spacing between the electrodes, which tends to geometrical change in the device.

Figure 9. Average capacitance values of (a) un-poled and (b) poled device measured at various loads. Figure 9(a) shows the change in capacitance with respect to the applied load in the developed composite film containing device. The capacitance was measured at 1 kHz and five


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times recorded for each data point, and average capacitance value was reported. It is observed that the initial capacitance of PVDF-CZnO composites (~ 768 pF) was substantially higher than PVDF-HZnO composites (~ 609 pF) and pure PVDF (~ 490 pF) and was supported the dielectric results. The significant change in the capacitance was observed with increasing the load up to < 0.002 MPa, and thereafter showed slight increase in capacitance. The educed slope of PVDFCZnO device is 3.98×10-8, which is higher than PVDF-HZnO composite when the applied pressure is < 0.002 MPa. Notably, both the devices containing composite film attribute superior pressure sensing response than that of pure PVDF film based device. There is a decrease in sensitivity in both the composites when the static-pressure is > 0.002 MPa. The observed change is evidenced in the inset Figure 9(a), and shows the change in the capacitance (∆C/Co). Where, Co is the initial capacitance with no load and ∆C is the capacitance difference i.e. (C-Co) under static loading. In a recent report, it was forecasted that the phenomenon of piezoelectric exist under application of stress for the capacitive pressure sensing system, where the fillers embedded in the host polymer matrix.11 The abrupt change in the capacitance of the composite is attributed not only due to the spacing between the electrodes but also the induced polarization caused by the addition of fillers in the Polymer matrix. When subjected to an external pressure, the spatial charge separations/ electric dipoles in the composite stimulate the relative movement of the Zn2+ cations and O2- anions of fillers.



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Figure 10. Schematic representation of un-poled and poled device with polarization effect.

In order to understand the existence of the induced polarization due to piezoelectric effect, the devices were poled under corona with applied voltage of 15 kV and temperature 60 oC for 1 hour. The schematic representation of the poled/un-poled device is shown in Figure 10. Thereafter, the capacitance of poled devices was measured under applied load (Figure 9(b)) and compared with un-poled devices. Interestingly, significant change was observed in the poled device (PVDF-HZnO composite film) than compared to un-poled one. Also, the poled PVDFHZnO composite film based device evidences appreciable sensitivity up to load 0.006 MPa (Figure 9(b)) than un-poled one (0.0015 MPa). Application of strong electric-field causes orientation polarization of dipoles to the applied field direction, results the enhancement of capacitance value. However, the poled PVDF-CZnO composite film based device did not show significant change in the sensitivity than the un-poled one. This suggested that the adsorption of carbon on ZnO saturate the spatial charge separations/electric dipoles and hindered further polarization. Also, pure PVDF film based device is not exhibited significant change in the capacitance value under applied pressure due to the existence of polar nature, and thus only showed slight improvement in the initial capacitance than un-poled PVDF film based device. 24 ACS Paragon Plus Environment

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Further, the PVDF-HZnO and PVDF-CZnO device was subjected to sequential compressive stresses (0.65, 1 and 2 kPa), and the change in capacitance were recorded at 1000 Hz as shown in the Figure 11 (a and b).

Figure 11. Change in capacitance response of the device with respect to various cyclic compressive stresses: (a) PVDF-HZnO nanostructures and (b) PVDF-CZnO nanostructures; (c) Change in capacitance response subjected to bending (inset showing the schematic bending image of the device). The cycle consisted of three applied compressive stresses having 10 s holding and 60 s lasting time and the cyclic loading was repeated over a period of time. Both the device showed 25 ACS Paragon Plus Environment


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fast and stable response to the applied stress. However, the initial response to the load is higher in PVDF-CZnO than compared to PVDF-HZnO device, and suggested the sensitivity response in low pressure range (< 0.65 kPa). Also, the robustness of the flexible device was evaluated in different bending positions; the corresponding capacitance change was monitored and shown in Figure 11(c). The device shows increase in capacitance change upon bending due to both the contribution of tensile stress in outer side and compressive stress in the inner side. These stresses cause dimensional change (strain) in the composites, thus changes the electrodes spacing as well as microcapacitance (spacing between the fillers) of radial ZnO in the polymer matrix. In summary, the induced electric potential developed in the hollow radial ZnO based polymer composites improved the dielectric properties, electromechanical performance, and potential candidates for building flexible capacitive pressure sensing device. 4. CONCLUSIONS Hollow radial ZnO and carbon nanoparticles decorated hollow ZnO nanostructures were prepared by using a promising ambient temperature template-free chemical process. The developed particles was successfully characterized by using sophisticated analytical techniques for understanding the crystal structure, morphology and number of unpaired electron before and after modification of ZnO. CZnO heterostructures showed strong interaction with the PVDF polymer matrix, significantly improved the dielectric properties, and suggested strong interfacial polarization due to formation of pseudo-microcapacitor structures. The polymer composite films were used as a dielectric layer for fabricating pressure sensing device and the capacitance was increased by a factor of 1.2 times in PVDF-CZnO composite films than compared to PVDFHZnO composite film. Further, the composite device was poled under electric field and shown an increase in capacitive response under applied load than compared to un-poled device. Overall, 26 ACS Paragon Plus Environment

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hollow radial ZnO based composite device exhibited fast and stable response to the applied stress and can put forward to fabricate low cost capacitive pressure sensing device. Supporting information: FESEM images & EDX spectra of HZnO, CZnO and Carbon nanostructures were shown in Figure S1 & S2. Dielectric studies of CZnO nanostructures with various concentration of carbon were shown in Figure S3. Dielectric studies of PVDF-CZnO composites with various concentrations were shown in Figure S4. Temperature dependent conductivity plots were shown in Figure S5. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements: The authors acknowledge Niraj kumar and Pooja shere for their support during the work; DST, Govt. of India for financial support; DIAT for characterization support. References:

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